In conventional optics, a spatial filter makes it possible to obtain a luminous beam freed from any imperfections linked to diffraction phenomena (interference rings).
In integrated optics, the problem is somewhat different. In fact, this involves isolating from a wave guide structure one single mode and ensure that it is this single mode that is able to extend inside the guide film of the structure.
A wave guide structure generally consists of a buffer film, a guide film and an upper film all stacked on a substrate, the guide film having a real refraction index greater than that of the buffer and upper films. In certain cases, the upper film may be replaced by air.
Strictly speaking, one could expect that the isolation of a mode in integrated optics would be relatively simple. In fact, a light microguide may easily be calculated and embodied so as to merely have a single guided mode. But these factors rely heavily on two points.
First of all, in theory, a guided mode is a mode of nil losses if the propagation media are not absorbant.
In practice and in the best of cases, there are still losses of between about 0.05 and 0.2 dB/cm.
In parallel with this guided mode, there are still other possibilities concerning the light to extend inside an integrated optical structure; these are substrate modes whose theoretical losses are no longer nil; they then only undergo partial reflections in the wave guide and, because of this, one portion of the energy leaks out into the substrate and is thus lost.
These substrate modes still exist in theory and according to the microguide structures in question, their theoretical losses able to vary between one fraction of dB/cm to several tens of dB/cm. This means that over short distances (100 .mu.m to several millmeters), their attenuation remains slight. They thus generate stray light which, in certain applications, may be extremely disadvantageous.
For certain microguide structures, in which the shape and dimensions of the microguide are fixed by etching of the upper film (structures known under the name of "rib waveguide" or "rib channel guide"), the light may also be planary-guided outside the microguide. This is the case with Si/SiO2/Si3N4/SiO2 structures developed in the electronic laboratory of the Applicant.
In these structures, the light remains confined in the effective high index region (that is, under the etched upper film), but it may also be guided into adjacent regions and especially when injection of light into the microguide is not perfect.
The second point posing a problem is as follows:
In fact, in practice it is not possible to inject light into a monomode wave guide structure and only excite the mode guided into this structure.
In order to do this, it is necessary to inject a distribution of luminous amplitude (by means of a monomode optical fiber or with a laser diode) strictly identical to that of the guided mode of the structure and superimposed on the latter. However, this is impossible to embody technically.
Moreover, certain integrated optical components themselves generate stray light by virtue of their instrinsic defects (surface roughness, punctual defects, etc). Also, even if the injection of light into the microguide were perfect, this second problem would still exist. Thus results in the following drawbacks:
a) increase of the stray light percentage, PA1 b) instability of the optical signal as this stray light may interfere with the guided light. Owing to the different effective indices involved, external interference does not act identically on the phase of the guided mode and on that of the stray light causing continuous fluctiations.
This problem is mainly identified on quiet complex devices sensitive to stray light, such as interferometric devices (displacement sensor, integrated optical gyrometer).